In some antenna systems with steerable beams or directional capabilities, such as active phased array radar antennas or active telecommunication base-station antennas, numerous antenna elements are arranged equidistantly in a plane whereby each element is adapted to emit and receive microwave signals.
In those systems, typically a microwave exciter and receive unit is provided for generating and receiving a signal. The exciter and receive unit is connected to a branch like transmission network, through which signals are emitted and received. The transmission network connects to respective transmit and receive modules, TRM, comprising electrically controllable phase shifters and amplifier stages through which the amplitude and phase delay of signals can be controlled. The transmit and receive modules are connected to the antenna elements. Typically, dipole elements may be used as antenna elements.
When signals are provided simultaneously to the plurality of elements, a plane wavefront parallel to the plane of the array is generated because of the in-phase interference of individual signals. When the phases of signals are incremented for each antenna element with regard to an adjacent element, a wavefront is propagating at a non-parallel angle with regard to the plane of the elements, which angle is dependent on the incremental phase delay. The elements may also be arranged at non-equidistant intervals, but then the individual delays are correspondingly controlled to provide a plane wavefront. By arranging a plurality of elements on a plane and controlling the emission with regard to two directions, the resulting direction of the emitted beam and the sensitivity of the received signal may be controlled in three-dimensional space.
One disadvantage associated with known active antenna systems is the amount of hardware required. A phased array antenna may for instance have several thousand individually controllable antenna elements.
Moreover, it is important that all individual transmission paths are of the same or of known length to accomplish the desired beam-steering control over the desired bandwidth.
An important characteristic of an antenna system with high sensitivity is the directional properties as expressed by the level of the side-lobes compared to the level of the main lobe.
For instance for airborne radar systems, such as Airborne Early Warning (AEW) systems, the side-lobes must be so well attenuated that unwanted ground and sea clutter can be efficiently suppressed. Low sidelobes are also required in order to suppress signals from other emitters in the neighbourhood such as signals from active hostile jamming. The low sidelobe level specification necessitates a tight control of the amplitude and phase of each transmit/receive module, TRM. When transmitting, the amplitudes of all TRMs have identical settings, whereas amplitude tapering is applied in receive mode. In air-cooled systems, the phase and amplitude control must cope with the large temperature variations that may prevail. This particularly applies to air-borne radar systems. For instance the feed and receive network may be subject to thermal expansion/contraction, which alters the phase of individual signals. One example of AEW system has been shown in U.S. Pat. No. 4,779,097.
Generally, antenna systems are complex systems with many components, which require accurate control.
In a distributed transmission system, utilising microwave wave-guides, the transmit and receive modules account for a majority of the errors that are introduced. Careful design of these parts with respect to long-term stability of performance, supply voltages, internal heating and ambient temperature is necessary but often not sufficient. Therefore, a need has arisen as to be able to calibrate antenna systems during operation.
In FIG. 1, a known antenna system has been shown. The system comprises an exciter/receiver unit ERU, a plurality of dipole antennas D1–Dk, respective couplers Q1–Qk being arranged adjacent the respective dipole antennas, a feed and receive transmission network (R) connecting the exciter/receiver unit and a plurality of T/R modules TRM1–TRMk, another feed network, AF, and a calibration network, C1.
The 1-k antenna elements may be evenly dispersed over a rectangular plane in a pattern of rows and columns.
In FIG. 1, a subset of the antenna elements, for instance D1–Dk, corresponding to a first row (or column) has been shown for simplicity. It should be understood that typically many more elements would form the first row and that subsequent elements up to element Dkk would correspond to additional rows.
The exciter/receiver unit, ERU, has a data bus XD, over which the exciter receiver unit controls the individual transmit and receive modules TRM for obtaining the desired directional capabilities.
Each respective T/R module has a feed AF that leads to an antenna element. The calibration network C constitutes a branch like structure with equally long distance to each respective coupler Q1–Qk. Calibration signals are sent through a port X, of the ERU, returning through a selected transceive and receive module TRM and returning through the first feed network R back to the ERU over the transmission network, R. The phase and amplitude of the signal is compared to a fixed reference for a given path. This procedure is completed for all transceive and receive modules, TRM.
Prior art document U.S. Pat. No. 5,412,414 shows a similar phased array radar system providing in-operation calibration. The radar system comprises an exciter, a receiver, a transmit/receive transmission network, T/R modules and dipole elements. Respective directional calibration couplers are provided adjacent the dipole elements for transferring signals through these to/from a calibration network which is different from the transmit/receive network. By issuing transmitting calibration signals from the exciter and leading signals through the transmit/receive transmission network and through individual T/R modules to couplers adjacent selected dipole elements, and return through the calibration network, variations in the transmit/receive network and associated components can be analysed. Likewise, by issuing receiving calibration signals from the exciter and leading signals through he calibration network to couplers adjacent selected dipole elements, and return through T/R module and the transmit/receive network, variations in the transmit/receive network and associated components can be analysed. One drawback with the above system is that an initial calibration, using external measurement equipment, seems to be required before in-operation calibrations can be carried out.
U.S. Pat. No. 5,874,915 shows an AEW phased array system having a plurality of selector switches for coupling a respective low noise receive amplifier or transmit amplifier to one of three antenna elements in a respective column of the antenna array.